Carbon fiber reinforced composites (CFRC) result in significant reductions in weight due to their lower density (1.5-1.8 g/cm3) when compared to aluminum alloys (2.7 g/cm3). CFRC are particularly of interest in the manufacture of vehicles, including airplanes and land vehicles such as cars and trucks. Furthermore, the use of composites simplifies manufacturing processes and leads to stronger components with higher durability. Expected increases in fuel costs will continue to motivate the use of composites in vehicle designs in the future.
Despite having clear advantages in weight reduction and fuel savings, the use of resin-based composites in modern vehicles presents new engineering challenges. Some of these stem from the dielectric nature of the resins that are used to prepare the CFRC. Traditional metallic components have high conductivity (Aluminum ˜3.6×107 S/m). Carbon fibers in CFRC significantly increase the effective conductivity (CFRC ˜1.5×104 S/m) but, because they are buried inside the material, large currents can cause structural damage. For these reasons, electromagnetic effect (EME) management of CFRC in vehicles (particularly airplanes) is especially important.
In particular, the mitigation of risks associated with lightning strikes is of high relevance to aircraft design. It is estimated that FAA-approved commercial airplanes are struck by lightning an average of two times every year. The primary lightning event (main stroke) requires dissipation of up to 200,000 Amperes over sub-millisecond timescales. When a suitable conductive path is not present, mechanical damage, thermal degradation and/or damage to electronic components can result. Moreover, lightning-related events such as corona discharge, streamers, and continuing currents can also persist before or after the main stroke exits the airplane. These events can result in serious damage to physical and electronic components even when not in the exit path of the main stroke. For example, continuing currents can also be significant (up to 200 Amperes) and need to be dissipated effectively.
The other main EME problem of importance is static charge buildup during normal flight conditions. Static charge can originate from the impact of airborne particles, rain or snow (i.e. triboelectric charging) or from the flow of hydraulic fluids or fuel. Static charge buildup hinders communications, interferes with electronic equipment and can lead to sparks and explosions in the presence of flammable vapors. The increased use of electronic navigation systems in modern aircrafts (e.g. fly-by-wire) that may be affected by static buildup further motivates the need for effective charge dissipation.
Current strategies for lightning and EME management in planes containing CFRCs consist of incorporating a conductive metallic mesh (e.g. Cu or Al) between upper plies of the composite. This allows effective current dissipation along the surface of the plane without penetrating deep into the composite material. Although this is an effective damage prevention strategy, it can add significant weight to the plane (Cu density is 8.9 g/cm3), reducing the magnitude of fuel savings. For this reason, metallic meshes are only added to critical sections of the planes such as those with high probability of lightning strike or where damage can be critical (e.g. fuel tanks).
A more powerful mitigation strategy that is also being explored is the use of conductive finishes (i.e. coatings) on the upper surfaces of the plane.
Because of their location, sacrificial conductive coatings can potentially dissipate enough electric current to prevent more serious damage to underlying structural and electronic components. Although lightning will irreversibly damage the coatings, these can be easily removed and reapplied. In contrast, damage to composite parts requires full replacement of the affected area at a much higher cost. Current commercial conductive finishes are usually composed of silver or copper particles dispersed within epoxy, acrylic or polyurethane carriers. The use of metallic particles leads to low sheet resistances (˜0.1 Ohm/sq for 0.05 mm thickness) but the creation of a connected conductive path (percolation) requires very high particle loadings (>50 wt %). This also translates to very large mass densities (>4 g/cm3) for the resulting coatings, adding to the total aircraft weight and reducing fuel savings. More importantly, the high particle loading requirements significantly deteriorate the mechanical and adhesive properties of the coatings so that they may not meet aerospace requirements.
Ideally, one would create conductive finishes that have low sheet resistances, low mass densities and which do not affect the adhesive and mechanical properties of existing coatings that have been optimized for this application. Conductive nanomaterials have been proposed as possible conductive finishes. The dispersion of conductive nanomaterials including carbon nanotubes, graphene, and nanoparticles into organic resins has been explored in order to modify the electronic properties of composite materials. Although some of these strategies show substantial promise, there are also significant problems preventing their application in conductive finishes. For example, carbon nanotubes have low percolation thresholds (˜0.5 wt %) and show significant increases in conductivity at higher concentrations (e.g. ˜0.2 S/m at 1 wt %). However, these changes are also followed by large increases in viscosity that makes coating difficult. There are also concerns about the toxicity of nanotubes and the potential for stronger regulation in the future.
Therefore, improved conductive finishes are desirable in order to advance the production of CFRCs and similar technologies.